If you get the flu, COVID-19, or another illness, you might experience a fever — an unpleasant, but temporary, increase in body temperature. It may surprise you to learn that getting a fever can impact sperm health and fertility for long after you feel better.
Key takeaways:
What is a fever?
A fever is a temporary but significant increase in body temperature. It could be considered a fever if you have a body temp of 99–100ºF or higher (normal is around 98.6ºF). A fever is not an illness in itself but is instead an immune system response; your body creates a fever to kill viruses, bacteria, or other pathogens that may be in your body. In a way, a fever is a warning sign that something is wrong internally. A few common causes for fever include the flu, pneumonia, COVID-19, heat exhaustion, food poisoning, and infections.
No matter how healthy your lifestyle is, you’re bound to get a fever at least once or twice in your life. So, how does getting a fever impact your fertility? Does a fever kill sperm? Does getting a fever cause any permanent damage to your sperm quality or production?
How body heat affects sperm production
First, a short biology lesson.
Sperm production, known as “spermatogenesis,” is a process that’s sensitive to heat. Ideally, sperm production occurs around 93.2ºF, which is about 5ºF below normal body temperature. This is why the testicles hang below the body, in the scrotum — to keep the temperature a bit lower. (It’s also why the “height” of your scrotum may change on cooler and warmer days.)
When the temperature of the scrotum or testicles increases, which can happen when you get a fever, sperm production may be impaired significantly. This can lead to fewer sperm and lower-quality sperm with more genetic abnormalities.
There is extensive evidence that increased scrotal heat can impact sperm production and decrease sperm count and quality. In one study, it was found that scrotal heat exposure led to severe reproductive impairment, including increased levels of sperm DNA damage and apoptosis (cell death). Another study that observed the effects of increased heat in the testicles found that even a mild rise in testicular temperature significantly increased abnormalities in the DNA inside sperm and significantly decreased normal morphology, a result of disruptions in spermatogenesis.
How fever affects sperm quality parameters
Research supports the conclusion that febrile illness — a sickness that comes with a fever — can significantly reduce sperm count and quality, especially when it’s a fever over 101ºF experienced for multiple days.
Fever and sperm count
Sperm count refers to the total number of sperm in the semen; sperm concentration is the number of sperm per milliliter of semen.
In a study that observed five men who had fevers of 102.2ºF or greater for 3 or more days, it was found that sperm concentration declined up to 7% within 6 weeks following the fever. In one case report, it was found that total sperm count was significantly decreased for up to 2 months after a 2-day fever of 102–104ºF. Another case report of an infertility patient had a similar outcome: sperm concentration dropped to less than half of its original measure just after the fever was resolved.
Another study followed 27 healthy, fertile men with monthly semen analyses. This research reported a 32–35% decrease in sperm concentration for men who experienced febrile illness. This study also found that the number of days the men experienced a fever significantly affected their semen parameters, with a reduced sperm concentration of up to 8.5% per day of fever.
Fever and sperm motility
Sperm motility refers to the percentage of sperm that are moving or “swimming.”
In one case study, it was observed that sperm motility was decreased for over a month post-fever. This observation is supported by the aforementioned study analyzing the effects of fever on 27 fertile men, which found that the percentage of immotile (non-moving) sperm was increased by 20.4% following fever — with an increase of 4.5% per day of fever.
The previously mentioned study of five febrile men also found a decrease in what’s known as “progressive motility,” the ability of a sperm to move efficiently (i.e., in straight lines or large circles as opposed to tight circles). There was a decrease in progressive motility of up to 23% around 6 weeks post-fever.
Fever and sperm morphology
Sperm morphology refers to the percentage of sperm that are the proper size, shape, and structure.
Sperm morphology is also decreased by an experience of fever. Research demonstrates that the percentage of morphologically normal sperm may decrease by over 7% post-fever. One case study was able to get even more specific, finding a significant increase in the number of sperm with abnormally small heads following a fever.
Fever and sperm DNA fragmentation
Sperm DNA fragmentation is a measure of the percentage of sperm that contain damaged or abnormal genetic material.
One case study analyzed the semen sample of a fertile patient who had the flu with a 1-day fever. The samples were analyzed at 18–66 days post fever. At 18 days post fever, 36% of the sperm analyzed demonstrated damaged DNA — considered a high, abnormal level of sperm DNA fragmentation. Another case report found severe damage to the DNA structure in a fertile patient with a 2-day fever of 102.2ºF–104ºF.
Can getting sick affect my fertility?
Yes, it’s certainly possible. Illnesses with fever can temporarily, but significantly, affect your fertility. This may especially be true for people who already have low sperm count or motility, as any additional decrease can further impact chances of conceiving.
Several of the above pieces of cited evidence looked at real-world cases of febrile illness, such as influenza or strep throat, and found that they indeed caused a drop in sperm count and quality. Other febrile illnesses such as COVID-19 have been shown to significantly impair sperm production, at least in the short-term.
Learn more about COVID-19 and male fertility.
However, a short-term or mild fever is not likely to have a significant effect. In one study, researchers found no impact on sperm production with a scrotal temperature increase of less than 2ºF. In another, researchers found that the impact was directly related to how long the fever persisted, with fevers lasting several days having a much more dramatic effect on sperm quality than shorter fevers lasting less than a day.
Can getting a vaccine affect my fertility?
Some vaccines, such as the COVID-19 vaccine, may cause a slight fever after injection. This is part of the normal immune system response to vaccination. However, the fever caused by vaccination is much shorter and milder than the fever that’s possible with infection, and is less likely to impact sperm.
Multiple small studies of the COVID-19 vaccine, for example, have shown that vaccination has no impact on sperm count or motility.
How long will it take my sperm to recover from a fever?
The good news is that the effects of a fever on sperm production are not permanent. You’re producing sperm all the time, and the spermatogenesis cycle takes around 72 days.
One study of scrotal heating noted that the effects on sperm production were reversible, and that normal sperm production could be recovered after two cycles of spermatogenesis. Case studies of patients with febrile illness have reported that normal sperm production resumed after anywhere from 20 to 45 to 79 days post-fever.
So, if you’ve recently had a bad fever, you can expect your sperm quality and production to return to pre-fever levels about two to four months post fever.
Should I do a semen analysis if I recently had a fever?
We don’t recommend it, because it won’t be the most accurate analysis. If you’ve recently had a fever, it’s probably a good idea to wait until at least two months after your fever has subsided for the most accurate results. Otherwise, you may be looking at sperm parameters that have temporarily changed due to the fever.
For the most accurate semen analysis reading, you may also want to abstain from ejaculation for 2–5 days before producing your sample. To learn more about how to prepare for a semen analysis test and what to expect, check out our sperm testing guide here.